The secret of great comedy, runs the old saw, lies in the timing. The same goes for photography, where split seconds make all the difference. Which must make Man Ray, one of the greatest photographers ever, supremely lucky. His career began when the popular view was, in one of his own exhibition titles, "Photography is not art". Philadelphia-born Michael Emmanuel Radnitzky only gave up painting and turned into Man Ray in 1920s Paris. And the people who sat for him! The retrospective at the National Portrait Gallery (closing this Monday) lines up a who's who of European modernism: Joyce, Stravinsky, Schoenberg, Satie (twinkling away like a kindly bank manager). By the end of the 20s, he was raking light over subjects such as Salvador Dali. After the war, Ray's portraits of Hollywood stars such as Ava Gardner defined the period. But his talent was now more than timing; he had helped establish photography as an art.